Archaeological evidence that a late 14th-century tsunami devastated the coast of northern Sumatra and redirected history

Significance We demonstrate that a tsunami in the late 14th century CE destroyed coastal sites along a critical part of the maritime Silk Road and set in motion profound changes in the political economy of Southeast Asia. Our results provide a precise chronology of settlement and trade along a historically strategic section of the Sumatran coast and are robust physical evidence for the rise of the Aceh Sultanate. Tragically, coastal areas impacted by the late 14th century tsunami were devastated by the 2004 Indian Ocean tsunami. This makes our findings relevant to debates about hazard mitigation and risk reduction. This example shows that archaeological, historical, and geological data are relevant in discussions about the long-term sustainability of communities exposed to geological hazards.


Gravestone Chronology
We located and documented 5,863 gravestones during the survey. For this paper, we are able to fit 1,866 gravestones within a crude chronology based upon comparative analysis with dated, published gravestones from the region and in a few cases, graves from our survey area that have been dated based upon inscription (1 -5). On this basis, we have identified 152 gravestones that most likely date to the 15 th century, another 116 from the late 15 th century through the early 16 th century, and 1,598 from the mid-16 th through the 18 th century (Table S4). Many of the undated gravestones are simple river-rounded stone markers and non-descript standing slabs that are impossible to directly date. A large percentage of the rest of the currently undated stones most likely belong to the Aceh sultanate period (dating from the early-16 th century), but more work is needed to refine the gravestone typology and chronology. The categories we use in this paper are sufficient for testing our hypothesis about possible changes to settlement caused by the 1394 and 1450 tsunamis.

th Century Gravestones
We found three types of gravestones within the survey area that we believe were produced during the 15 th century. There is a vernacular style of Muslim gravestone, locally known as plang pleng, that is seemingly unique to this section of the Aceh coast. Plang plengs are standing squared pillars that taper into a pyramidal apex, usually less than 1 meter in height.
All known examples have ornamentation, which can include elaborate floral and/or geometric patterns, and Arabic inscriptions, generally contained within panels near the base of the gravestones ( Figure S7). Several clusters of plang plengs were previously analyzed by scholars, with 11 containing the date of burial within inscriptions (Table S5) (1). Therefore, we propose this style of gravestone dates from the early 15 th through the late 15 th century. We don't know of any plang plengs that have been dated later than the 15 th century.
We recorded a number of standing squared pillar gravestones that that share some characteristics, but are distinct from the plang pleng tradition in terms of form and style. While we have not yet obtained any direct dating from the inscriptions on them, they share some characteristics with dated gravestones from Pasai ( Figure S8) (1). The examples from Pasai date to 1415 and 1483-4. Based upon stylistic comparison, we feel it is likely that the examples we recorded in our survey zone also date to the 15 th century, and are most likely indicative of some form of cultural connection between areas in our survey zone and Pasai during the 15 th century.
We recorded a number of standing slab graves with inward curving sides. While most of these stones are blank, we recorded several that have elaborate ornamentation. The carved gravestones have very clear stylistic similarities with several dated sets of gravestones from Pasai that date between 1430 and 1440 ( Figure S9) (1,5). We therefore feel it is likely that the examples we recorded in our survey zone date to the 15 th century and are further evidence for cultural connection between residents in our survey zone and Pasai.

Late 15 th -Early 16 th Century Gravestones
We recorded two types of gravestones that we suspect date between the late 15 th through early 16 th century. Because some of the dated comparative examples from Pasai span the two centuries, we have put them in a distinct chronological category that overlaps with the 15 th century group. However, we have yet to find any examples of these gravestones that we can date conclusively before the late 15 th century.
We recorded four thick standing slab gravestones with ornamentation and inscriptions that are virtually identical in style to gravestones from royal tombs in Pasai ( Figure S10) (1).
Dates from inscriptions in Pasai from stylistically similar gravestones range from 1510 to 1519.
We therefore feel it is likely that the examples we recorded in our survey zone date to the early 16 th century and are further evidence for cultural connection between residents in our survey zone and Pasai.
We recorded gravestones that we initially categorized as plang pleng grave markers based upon stylistic similarities. More detailed analysis has led us to suspect that there is a later type of plang pleng that dates from the late 15 th through early 16 th century. This is based upon several observations: 1) some plang plengs are much more homogenous in terms of physical characteristics than the types of plang plengs that have dated inscriptions; 2) the 'later' plang plengs are not found in association with any gravestones that are dated to the 15 th century, but are found in association with graves dated to the late 15 th -early 16 th century (and later); and 3) we have found several unique examples of graves which seem to share characteristics of the 'later' plang pleng style and some of the early batu Aceh gravestones ( Figure S11). This leads us to speculate that some types of plang plengs are younger than the dated plang plengs and fall into the period immediately before the Aceh sultanate developed. We did not include these grave sites on the map in the main text because our dating is highly circumstantial and not yet robust enough to go beyond speculation.

Mid 16 th -Late 18 th Century Gravestones
There are well-documented traditions of gravestones associated with the Aceh Sultanate called batu Aceh. These vary considerably in terms of size, shape and ornamentation and are found throughout areas that fell under the Aceh sultanate, including northern Sumatra, and the Malay/Thai peninsula (6 -7). Several of the main styles of batu Aceh have been dated on the basis of inscriptions, with the main period of their production spanning the mid 16 th century through the 18 th century.
One of the most distinctive styles of batu Aceh are 'winged' slab stones, often with extensive ornamentation and inscriptions. Within this broad category we found an extensive variety of stones ( Figure S12). Based upon comparison with dated published gravestones, we consider these to date from the mid 16 th century through the 18 th century. Published accounts demonstrate that there are early examples of 'winged' slabs that date to the 15 th century from Pasai (5 -7). However, we did not identify similar stones in our survey zone, leading us to conclude that only later versions of this style of gravestone are present within our survey zone.
We recorded squared pillar stones, with elaborate bases, extensive ornamentation and multi-tier finials that are associated with the Aceh sultanate ( Figure S13). Published dated examples suggest that this style dates from the late 16 th century, possibly running through the 17 th century (2 -4).
We recorded vertical slab gravestones, with curved shoulders, multi-tier finials, and often elaborate ornamentation that are associated with the Aceh sultanate ( Figure S14). Published dated examples suggest that this style dates from the mid to late 16 th century (4).
We recorded gravestones that consist of vertical columns with six or eight edged faces, some with ornamentation. Several similar gravestones have been are associated with the Aceh sultanate ( Figure S15). Published dated examples suggest that gravestones in our survey area that are of this style date from the late 16 th century (3 -4).

Ceramics Chronology
We dated the trade ceramics recovered during our survey based upon dated reference collections obtained from shipwrecks, archaeological investigations, and museum collections. We were able to assign a geographic source and date range of production to 61% of the ceramic sherds found in our survey area. Working with the ceramic data is complicated by the wide variety of date ranges for different types of ceramics. We recovered several sherds that we could date precisely to a specific year (based upon a stamped date). However, most of the sherds had date ranges that spanned decades, and in some cases, centuries.
We applied a statistical model to the ceramic dataset to calculate probable activity levels for Fig. S1. Exposure of cultural layers and natural deposits in a beach cliff below the elevated headland in Cluster 10 (Lamri) revealed two centuries of occupation before destruction of a small structure by a tsunami soon after 1366±3 C.E. A 14 th -century lime plaster floor (yellow) covers a 13 th -to 14 th -century rubbish pit (gray). A layer rich in foraminifera tests (a form of marine microfossil) and coral (pink) overlies the floor, providing evidence of marine overwash consistent with a tsunami. The uppermost layer is a man-made earthen ridge built parallel to the cliff. Blue squares, red triangles, and the black star correspond to samples with radiocarbon ages (black horizontal lines) at corresponding dates on right. The red lines indicate the two uplift events documented by Meltzner et al 2010 (13). From Sieh et al 2015 (14).  Counts of all material, including sherds for which the production date can range across more than one of the time periods. These materials are counted in each of the periods during which they might have been produced. Figure S3B thus gives a maximum limit for sherd counts for each period. Counts of all material, including that which is not constrained within one of the time periods. These materials are counted in more than one period. Thus Figure S5B gives a maximum limit for sherd counts for each period.         Supplementary Tables   Table S1. Quantity of ceramics sherds dated with confidence by period for each cluster. Showing total count of entire assemblage, count of all sherds of 'high class' fine wares, and count of sherds of large jars.